The present invention relates to locking threaded fasteners, and more particularly to such fasteners that can be driven using conventional driving tools.
Many locking fasteners have been developed to increase the reliability of threaded joints. For example, several known locking fasteners include split, star and spring locking washers, ramped washers, thread adhesives, wire locking pins, cotter pins, external bracing, ratchet devices, jam nuts and polymer thread patches. Other known locking fasteners include lock nuts with nylon or metal inserts, distorted thread profiles and/or oval female thread diameters. These locking fasteners are widely used in applications involving cyclic loading, vibration, thermal expansions and flexing of one or more joint members. They can be used across all industries, but are essential in demanding fields such as surgical implants and fixation systems, automotive, aerospace, racing, off-road and construction equipment, turbine and power generation, electrical connections and any equipment subject to vibration and thermal cycling.
Some of these locking fasteners, for example lock washers, are widely used but have been shown to be unreliable with prolonged exposure to cyclic loads and vibration. Other locking fasteners, while effective, can be comparatively expensive. For example, Nordlock® ramped washers consist of a stacked pair of hardened steel washers with serrations on the joint face and interlocking ramps to prevent the bolt from backing out. While effective, the ramp washers are comparatively expensive and two sets must be used if both a bolt and a nut are used.
Thread adhesives, such as those produced by Loctite Corporation, are effective when applied in a controlled environment to properly prepared surfaces. However, thread adhesives are best suited for permanent joints where frequent removal and reinstallation are not required. In addition, wire locking and cotter pins are safety measures which prevent fasteners from separating from the joint but do not prevent the fastener from loosening. External bracing is a specialized solution, and is often customized to a particular application. Like thread adhesives, wire locking, cotter pinning and external bracing are inconvenient with respect to removal and reinstallation.
Fasteners with ratchet devices having detents and pawl arrangements that lock the nut at fixed angular positions on the male thread are available for some applications, but are complex and not widely used. In addition, jam nuts are sometimes used to back up a primary nut. However, jam nuts have the effect of reducing the tension between the bolt and the primary nut which may adversely affect the joint strength.
Prevailing torque locking fasteners create a prevailing torque which acts both during installation and removal. Prevailing torque inhibits self-loosening, but it slows installation and requires specialized tools for installation because the fastener cannot easily be turned by hand. Prevailing torque must also be accounted for when setting joint torque specifications. Since prevailing torque can vary widely and diminish with reuse, maximum joint torque specifications must be de-rated to prevent applying too much joint torque if using a low prevailing torque fastener such as a reused fastener. Prevailing torque is created through thread interference, which increases friction. Wear occurs each time the fastener is used and reused, which reduces the prevailing torque. Distorted threads and oval female threads also have limitations because they can damage mating threads, making them unsuitable for use in frequently serviced equipment, such as race cars, aircraft and heavy equipment, for example. Galling of stainless steel fasteners can occur when using and reusing prevailing torque fasteners.
While the above fasteners can be effective, there is an ever continuing need for fasteners with improved effectiveness and cost efficiency. For example, several particularly effective self-locking threaded fasteners are disclosed in U.S. application Ser. No. 11/955,736, filed Dec. 13, 2007 and entitled “Locking Threaded Connection,” and U.S. application Ser. No. 12/425,711, filed Apr. 17, 2009 and entitled “Locking Threaded Fastener,” which are hereby incorporated by reference in their entireties. These applications disclose locking fasteners having an increased holding power while simultaneously overcoming many of the limitations discussed above. The disclosed locking fasteners can include a helical coil locking element, formed from round wire, providing continuous one-way torque and acting to prevent loosening of the nut but having no effect on installation torque. The locking element, located in a counterbored pocket in the nut, mates with and nests in the male threads and operates by gripping the male thread tightly when a loosening torque is applied to the joint. No thread interference friction is created. Since there is no increase in thread friction, there is no significant wear and the nut can be removed and reused without loss of holding power. Since there is no installation prevailing torque, the joint torque specification does not have to be reduced to account for a range of prevailing torques.
The unique, continuous one-way torque characteristic provides locking at any angular position of the nut on the male thread. Under cyclic loading, this characteristic allows the nut to resist loosening torque while in some applications the nut actually tightens under each half cycle of torque in the tightening direction. By contrast, standard prevailing torque locknuts resist loosening torque but cannot re-establish tightness in response to cyclic loading. However, the helical locking element is anchored to the nut at one end and free on the other end. It is installed in the nut such that as the male thread is engaged, the coil expands so that the male thread can slip relative to the coil. If a loosening torque is applied between the nut and male thread, friction between the coil and male thread causes the coil to tighten its grip on the thread under loosening torque.
In order to prevent the helical element from breaking when the nut is intentionally removed, the free end of the helical element can be bent such that a standard wrench engages it as the nut is torqued off the male thread. This releases the gripping torque and load on the anchoring tang so that the nut can be removed using standard wrenches without damage to the helical locking element. Since the release tang can assume different positions when used with male threads having a range of functional diameter tolerances, a novel release ring is also disclosed which adds utility by allowing the nut and standard wrenches to be used over the full range of male thread functional diameter tolerances.
Despite the advantages of the locking fasteners disclosed in the above referenced applications, a major need exists for locking screws that can be used interchangeably with standard screws. For cost reasons, it is generally preferred to use screws in threaded holes rather than bolts with nuts. There are countless examples of this in a wide variety of industries. For example, surgical fixation plates are commonly attached directly to bone using surgical screws. In automotive and industrial batteries, it is common to attach cables with bolts screwed into holes incorporated in the battery terminals. It is common for these screws or bolts to use polymer locking patches or adhesives to prevent loosening due to cyclic loading and differential thermal expansion due to the combination of materials used in the joints. New bolts are generally used each time an industrial battery cable bolt is removed during battery service. Battery manufacturers instruct users to re-tighten cable bolts on a regular schedule. Thus, there remains a significant need for a locking screw that can maintain its holding power for extended periods and allow multiple reuse cycles without degradation in performance.
In general, there is a need for internally and externally threaded fasteners that:                a) allows the use of standard installation and removal tools without the need for special release rings or devices;        b) provides locking torque equal to or greater than that provided by prevailing torque type locking screws and nuts;        c) maintains locking torque over multiple reuses;        d) can be applied to all standardized thread forms;        e) can be applied to any size screw and nut;        f) functions over the tolerance ranges for standardized fasteners;        g) provides substantially greater removal (locking) torque than installation torque (i.e. has continuous one-way torque); and        h) is manufacturable using mass production equipment.        
The above needs are partially satisfied by the fasteners disclosed in the above applications. For example, the helical locking element provides the desired locking torque levels and continuous one-way torque characteristic. However, there remains the need to apply this concept to screws and to eliminate the need for the release tang and release ring yet still provide for high locking torque without risk of breaking the anchor tang on the band when removing the fastener with standard tools.
In addition, the above applications only address nuts using a locking spring made with round wire having a diameter equal to or less than the pitch (lead) of the thread. This allows the coil to have sufficient turns so that it will grip the male thread without slip. Maximum locking torque then is limited to the shear strength of the coil anchor tang. Round wire coils are able to meet the Industrial Fastener Institute standards for prevailing torque type locknuts (IFI-124/524). But, in order to remove the nut without breaking the anchor tang, a release means has to be provided to relieve the force on the anchor tang. A primary need is a way to use the coil locking element in a nut without the need for any release device and to provide sufficient locking torque without risk of breaking the anchor tang on the band during removal.
It is also a need to apply the coil locking method to screws as well as nuts. The release method used on the nuts is not appropriate for use with screws. It is therefore necessary to find a way to use the coil locking element in a screw without the need for any release device.